DISTURBANCE ASSESSMENT AND MITIGATION OF GREAT BASIN RANGELAND
Location: Watershed Management Research
Title: Hydrologic vulnerability of Great Basin sagebrush-steppe following pinyon and juniper encroachment
Submitted to: Society for Range Management Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: October 5, 2010
Publication Date: February 6, 2011
Citation: Williams, C.J., Pierson, F.B., Kormos, P.R., Hardegree, S.P., Clark, P.E., and Al-Hamdan, O.Z. 2011. Hydrologic Vulnerability of Great Basin Sagebrush-Steppe Following Pinyon and Juniper Encroachment. In: Abstracts of the 64th Annual Meeting of the Society for Range Management, February 6-10, 2011, Billings, MT.
Extensive woodland encroachment into sagebrush-steppe has altered vegetation structure and hydrologic function of Great Basin rangelands. Tree encroachment elicits a coarse vegetation pattern of tree dominance and spatially well-connected sparsely-covered intercanopy area. These changes coarsen the cover structure that, under pre-encroachment conditions, conserves water and soil. Similar transitions have occurred in the Desert Southwest with grassland-to-shrubland conversions and intercanopy vegetation degradation on persistent woodlands. Coarsening cover structure has been linked to amplified overland flow and soil erosion. We evaluated relationships in cover reductions and hydrologic response for multiple Great Basin sagebrush-steppe rangelands encroached by pinyon and juniper. Runoff and erosion from rainsplash, sheet flow, and concentrated flow processes were significantly greater from intercanopy than canopy areas across small- (0.5 m2) to large-plot (13 m2) scales. Runoff and erosion were dictated by the type and quantity of basal cover. Litter protected the soil surface from raindrop impact, provided rainfall storage, mitigated soil water repellency, and promoted aggregate stability. Surface runoff and sediment yield increased exponentially where bare soil and rock cover exceeded 50%. Sediment yield was linearly correlated with runoff, but varied among sites based on site-specific erodibility. Measured runoff and erosion rates suggest that while tree encroachment creates stable canopy patches, reduced shrub and herbaceous cover promote overall cover coarsening and amplify runoff and erosion across at least 30 m2 areas. Results indicate overall hydrologic vulnerability of sagebrush-steppe following woodland encroachment depends on the potential influence of tree dominance on bare intercanopy expanse and connectivity and site-specific erodibility of intercanopy soils.